A motor for driving an actuator used such as for a throttle valve of an automobile engine and an optical axis controller of an automobile headlight requires high responsivity and durability against vibration and shock. Hence, a motor is devised with its responsivity and durability against vibration and shock improved by reducing the inertia moment and weight of the rotor with a steel plate used for the rotor yoke. (Refer to patent literature 1, for example.)
As a first example of a conventional motor, the structure of a motor described in above-described patent literature 1 is shown in FIG. 8. In FIG. 8, the stator is composed of yoke bracket 8001 and iron core 8002 fixed onto the inner circumferential surface of yoke bracket 8001, with a coil wound therearound. The rotor is composed of magnetic plate materials 8022, 8023 molded in a hollow, circular shape as a rotor yoke; magnet 8021, tubular, fixed onto the outer circumferential surface of magnetic plate materials 8022, 8023; and shaft 8006 fixed penetrating through the central portion of magnetic plate materials 8022, 8023. Both ends (two positions) of yoke bracket 8001 have bearings 8007 arranged thereon rotatably supporting shaft 8006 of the rotor.
In the motor described in above patent literature 1, the inertia moment is reduced with a magnetic plate material in a hollow, circular shape used as the rotor yoke to enhance the starting performance and responsivity, while supporting a lightweight rotor at two positions outside the rotor yoke to be resistant to vibration and shock.
However, in the conventional structure shown in above-described patent literature 1, the rotor yoke is bisected and combined, which causes impossible assembling of the rotor magnet and/or imbalance of the rotor due to the difference in the direction (phase) of outer circumferential deflection of the two parts.
FIG. 9 illustrates this problem. FIG. 9 shows a part of the rotor excerpted from FIG. 8, where a component same as that in FIG. 8 is given the same reference mark as that in FIG. 8.
In FIG. 9, when shaft 8006 is fixed by press-fitting into or bonding to magnetic plate materials 8022, 8023, both materials are very rarely fixed in a zero-degree perpendicularity, but are fixed with some inclination. The degree and direction (phase) of the inclination are different between magnetic plate materials 8022, 8023, and thus effective maximum external diameter D93 of the rotor is larger than respective external diameters D91, D92 of materials 8022, 8023. This causes impossible assembling of tubular magnet 8021 formed with its internal diameter slightly larger than D91 and D92 so as to fit with the rotor yoke.
The inclination of shaft 8006 from magnetic plate materials 8022, 8023 causes outer circumferential deflection of magnetic plate materials 8022, 8023, undesirably resulting in imbalance of the rotor.
As a second example of a conventional motor, the structure of another motor in practical use is shown in FIG. 10. Here, FIG. 10 shows a brushless motor in which electronic components are incorporated.
In FIG. 10, the stator is composed of frames 1410, 1420 and iron core 1210 fixed onto the inner circumferential surface of frames 1410, 1420, with coil 1220 wound therearound. Rotor 1100 is composed of rotor yokes 1121, 1122 molded in a cup shape, stacked in the same direction; shaft 1110 fixed penetrating through the central portion of rotor yokes 1121, 1122; and tubular magnet 1130 fixed to the outer circumferential surface of rotor yokes 1121, 1122. Shaft 1110 further has thrust ring 1140 fixed thereto, composing a thrust bearing together with bearing 1302.
This thrust ring 1140 provides space H101 in the direction of the shaft penetrating for incorporating electronic components on printed-circuit board 1510, while setting diameter D102 of a hole bored in printed-circuit board 1510 to a minimum diameter larger than diameter D101 of the thrust ring, thereby ensuring a mounting area for electronic components.
Frames 1410, 1420 have bearings 1301, 1302 arranged thereon supporting shaft 1110 of rotor 1100, where rotor 1100 is rotatably retained. 1521 is a magnetoelectric transducer for detecting the position of the magnetic pole of magnet 1130, mounted on printed-circuit board 1510 to transmit a signal representing the position of the magnetic pole of the rotor to the motor driving circuit (not shown).
With the structure of this motor, the problem same as that in above-described patent literature 1 occurs. That is, the rotor yoke is bisected and combined, causing impossible assembling of the rotor magnet and/or imbalance of the rotor due to the difference in the direction (phase) of outer circumferential deflection of the two parts. Besides, thrust ring 1140 is required additionally, thus increasing the number of components. Further, durability against vibration and shock becomes dependent on the strength for fixing thrust ring 1140 to shaft 1110, thus decreasing reliability.
[Patent literature 1] Japanese Utility Model Unexamined Publication No. H01-64969 (Japanese Utility Model Publication No. S62-158767)